This invention relates to an improved sealant for insulating glass windows which has a combination of the best properties of a polyurethane and a polysulfide. It further relates to a polyurethane sealant made from a hydroxyl terminated polysulfide polyformal.
The term xe2x80x9cinsulating glass windowxe2x80x9d refers to a structure comprising panes of glass, the faces of which are in spaced relationship, thereby providing a space between the panes which imparts insulating properties to the structure. In its most widely used form, 2 parallel panes are positioned in spaced relationship by metallic or organic resin spacers placed in board around the perimeter of the panes, thereby forming a U-shaped channel in which the interior faces of the pane are the legs and a side of the spacer is the base of the channel. Typically, the spacer is a hollow form filled with a water-absorbent material, such as a molecular sieve, to keep the enclosed air space dry. The U-shaped channel around the perimeter of the window is filled with a sealant which must have a combination of properties for satisfactory use. Some of those properties are as follows.
The sealant must have very low moisture vapor transmission (MVT) rate so that moisture is prevented from entering the dry space between the panes of glass. Moisture in such space tends to condense on the interior faces of the panes, creating visibility and aesthetic problems. If the sealant does not have a satisfactory MVT rate, the longevity of the insulated unit will be severely reduced.
The sealant should have good elongation and flexibility so that it xe2x80x9cgivesxe2x80x9d during contraction and expansion of the insulated glass structure caused by temperature changes, thus relieving glass stress.
The sealant should also form an excellent bond with glass which is not degraded over long periods of use when exposed to sunlight, moisture, and large temperature changes. Tensile adhesion strength is an important indicator of bond strength.
In U.S. Pat. No. 3,386,963, Santaniello discloses certain isocyanate terminated polysulfide pre-polymers that are curable with polyols to form sealants and adhesives. The pre-polymers are made from hydroxyl terminated polysulfides having the formula:
HOxe2x80x94Rxe2x80x2SS(Rxe2x80x3SS)nRxe2x80x2xe2x80x94OH
wherein Rxe2x80x2 and Rxe2x80x3 are the same bivalent aliphatic radical wherein the carbon chain may be interrupted by oxygen atoms and the molecular weight is from 500 to 9000. There is no teaching about the properties of polyurethanes having a polysulfide backbone when Rxe2x80x2 is polymethylene.
It is an object of this invention to provide a method for sealing insulating glass windows with rugged sealants that combine the excellent liquid polysulfide attributes of good resistance to solvents and wood preservatives, low argon migration, good resistance to free radical oxidation, and good workability with the polyurethane characteristics of low moisture vapor transmission (MVT), low water swell, good electrical resistance, and good adhesion to organic components of insulated glass windows.
It is another object of this invention to provide a sealant whose hardness is less than that of a polysulfide-based polyurethane sealant of the prior art having the same molecular weight and cross-link density.
It is a related object of this invention to provide an isocyanate-terminated polysulfide which may be used as an intermediate or precursor in the formation of the insulated glass sealant.